Acute infections of the respiratory tract are very common, especially in infants, children and the elderly. In the US, for patients five years or older, ARI's accounted for 8% of all visits to primary care and 58% of all antibiotics prescribed in primary care (Grijalva et al. JAMA 302, 758 (2009)). These infections produce conditions such as otitis, bronchitis, pneumonia, sinusitis, pharyngitis and strep throat.
Acute bronchitis, which results from an infection, is an inflammation of the bronchi. It is one of the most prevalent respiratory infections treated in primary care and occurs most often during the winter. It is an acute illness usually lasting less than three weeks with coughing as the main symptom and at least one other lower respiratory tract symptom such as wheezing, sputum production or chest pain. Older people can have symptoms such as confusion or rapid breathing. About 90% of cases of acute bronchitis are caused by viruses, such as rhinoviruses, coronaviruses, adenoviruses, metapneumoviruses, parainfluenza viruses, respiratory syncytial viruses and influenza viruses. About 10% of cases are caused by bacteria such as Mycoplasma pneumoniae, Chlamydophila pneumoniae, Bordetella pertussis, Streptococcus pneumonia and Haemophilus influenzae. 
Treatment for acute bronchitis usually involves just treating the symptoms, e.g., with non-steroidal anti-inflammatory drugs (NSAIDs) to treat fever and sore throat. As most cases of acute bronchitis are caused by viruses, antibiotics are not recommended but tend to be heavily overused. Over 60% of bronchitis patients are prescribed antibiotics, and this is a leading cause of the development of antibiotic-resistant bacteria. This over prescription occurs despite fact that there is no clinical benefit for most patients.
While acute bronchitis is generally not serious, serious complications can occur as a result. Pneumonia is the most common complication of acute bronchitis and occurs when the infection spreads further into the lungs, causing air sacs inside the lungs to fill up with fluid. 1 in 20 cases of acute bronchitis leads to pneumonia. Another possible complication is respiratory failure. It is also possible that acute bronchitis can lead to chronic bronchitis. These complications are more common in immune-compromised people such as the elderly, diabetics, etc. For example, the number of hospitalisations for acute lower respiratory infections in England is about three times higher in those over 75 years than in younger people. Further, the average length of stay for acute respiratory conditions increases progressively with age.
Although most ARI's are resolved reasonably quickly, the suffering and the financial costs of respiratory tract infections are enormous. Little effective prevention is currently possible outside of two strategies: attempting to avoid contact with or spreading of infectious agents; and vaccination for influenza and pneumococcal pneumonia.
Human milk oligosaccharides (HMOs) have been proposed as a possible means for preventing or treating ARIs. In WO 2012/076323, a method is disclosed for preventing or treating ARIs in children of up to three years old using HMOs. The children are treated using nutritional compositions which contain an N-acetyl lactosamine, a sialylated oligosaccharide and a fucosylated oligosaccharide. The method is in particularly suited for treating otitis or bronchiolitis. In WO 2014/100696, a nutritional composition is disclosed for modulating inflammation, such as a respiratory virus-induced inflammation, in adults and the elderly. In US 2015/0031645, a method is disclosed for improving the immune system's response to a viral infection, such as a respiratory viral infection, in children of up to twelve years old. A nutritional composition which contains 2′-fucosyllactose and lacto-N-neotetraose is disclosed as being preferred.
However, there remains a need for an effective, as well as convenient and safe, method for preventing ARI infections and/or relieving their symptoms in immune-compromised persons, particularly adults.